1) Field of the Invention
The present invention relates to a technology for judging accurately whether there is an abnormality in a head of hard disk.
2) Description of the Related Art
Conventionally, a disk array device that divides and writes data in plural hard disks has been known. For example, in a disk array device having first to fourth hard disks, when the disk array device receives data A, B, C, D, E, and F in order from a host computer, the disk array device writes the data A, C, and E in the first hard disk, writes the data B, D, and F in the second hard disk, writes the data A, C, and E in the third disk, and writes the data B, D, and F in the fourth disk.
Since the disk array device divides and writes the data in the first to the fourth hard disks in this way, a data transfer speed can be increased. In addition, since the disk array device writes the data, which are identical with those written in the first and the second hard disks, in the third and the fourth hard disks, even when the first and the second hard disks break down, likelihood of losing the data can be reduced.
In addition, the conventional disk array device checks heads included in hard disks periodically to judge whether data can be recorded in the hard disks accurately.
More specifically, the disk array device writes binary data called a data pattern in areas where the heads of the hard disks are checked. Then, the disk array device reads the data pattern and judges whether the written data pattern and the read data pattern coincide with each other to detect abnormality in the heads.
An input/output control device described in, for example, Japanese Patent Application Laid-Open Publication No. H7-306812 writes data in a disk, executes reading of the written data, and when a reading error occurs, judges that there is an abnormality in the head of the hard disk.
However, with the conventional techniques the abnormality in the head cannot always be detected accurately.
For example, the conventional method detects the abnormality in the head using one kind of data pattern. When abnormality occurs in a write head and the data pattern cannot be written accurately, data written previously is read out when the data pattern is read. In this case, since data pattern written previously is identical with the data pattern attempted to be written, the written pattern and the read pattern coincide with each other, and the disk array device misjudges that the write head is in a normal condition.
In addition, even when the abnormality in the head is detected from occurrence of a reading error, it cannot always be judged that there is an abnormality in the head because the reading error may result from a cause other than the abnormality in the head.